Nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, has a high energy density and may thus let a large current flow and generate heat in a case where a breakage in the battery or in the device using the battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous electrolyte secondary battery should prevent more than a certain level of heat generation to ensure a high level of safety.
Safety of a nonaqueous electrolyte secondary battery is typically ensured by imparting to the nonaqueous electrolyte secondary battery a shutdown function, that is, a function of, in a case where there has been abnormal heat generation, preventing passage of ions between the cathode and the anode with use of a separator to prevent further heat generation. More specifically, a nonaqueous electrolyte secondary battery typically includes, between the cathode and the anode, a separator that has a function of, in a case where, for example, an internal short circuit between the cathode and the anode has caused an abnormal current to flow through the battery, preventing that current and preventing (shutting down) the flow of an excessively large current for prevention of further heat generation. The separator is typically made of a filmy porous film whose main component is, for example, a polyolefin-based resin which melts at approximately 80° C. to 180° C. when abnormal heat generation occurs.
There has been known a technique of laminating a porous layer on at least one surface of a porous film in order to improve a function of a separator made of the porous film. For example, Patent Literature 1 discloses that, in order to prevent an internal short circuit of a battery, a porous film containing an inorganic filler and a film binding agent is laminated on a separator which is a microporous sheet made of a polyolefin-based resin. A porous film which is thin, uniform, and excellent in flexibility is realized by defining an 85-degree specular gloss of the porous film.
Patent Literature 2 discloses a technique in which a 60-degree specular gloss is defined with respect to a separator which is obtained by applying, to a polyethylene microporous film, a composition containing insulating fine particles and an organic binder, in order to prevent a short circuit and improve reliability.